1. Field of the Invention
The invention relates generally to pulsed high energy arc-tube flashlamps and more specifically to monocoque structures and re-entrant seals for pumped neodymium-doped glass laser flashlamps.
2. Description of the Prior Art
Very high power arc-tube flashlamps present several challenges to long life operation and acceptable low impedance levels. The National Ignition Facility (NIF) is due to be constructed by the United States Department of Energy (DOE) at Lawrence Livermore National Laboratory (LLNL) beginning in 1997. The NIF will use pulsed high energy arc-tube flashlamps to pump each of 192 laser beams that in turn are all converged on a target hohlraum loaded with a fusion fuel pellet. The solid-state lasers used to drive inertial fusion energy reactors must generate fusion-like, megajoule pulse output. Various conceptual and technological innovations were needed to overcome the limitations of the prior art, and thereby enable NIF design and construction to proceed.
A typical pulsed high energy arc-tube flashlamp for the NIF is a cylindrical quartz tube specified to mount in a laser cassette and have an overall length of about 2000 millimeters (mm), an arc gap length of about 1800 mm, and mounting ends that are about thirty-eight mm in diameter and ninety mm long. A large capacitor bank is used to deliver as much as 20,000 amps at 24,000 volts in a single thirty-two kilojoule pulse to each arc-tube flashlamp. Such currents generate tremendous magnetic fields and plasmas that hammer hard at the seals between the tungsten electrodes and the quartz glass envelope in the lamps. Prior art small diameter rod seal lamps and o-ring seal lamps have proven inadequate for such extreme applications.
Conventional lamps require external lamp mounting bases or structures, and so need extra parts that add to the overall cost. Such designs can be weak at the glass envelope necks of the pulsed high energy arc-tube flashlamp where the bending moments are concentrated. Prior art glass-to-metal seals in pulsed high energy arc-tube flashlamps have been complex and made manufacturing more difficult. Re-entrant seals, o-ring seals, and solder seals have been used in prior art designs, but none seems to have ever used electrode-to-glass seals that exceeded 0.125 inches. These small diameters appear to have been preferred to maintain electrode flexibility outside the tube envelope so that the seals themselves would not be over-stressed, but the seals are weak.
The NIF size and complexity has raised the cost of laser glass and laser down time to proportions that require the flashlamps to have zero explosions and zero insulation failures.
All of the highest energy laser facilities have been based on flashlamp-pumped neodymium-doped glass (Nd:glass) lasers because they are most amenable to large-scale deployment and they provide the flexibility in output characteristics needed for ICF and weapons-related experiments.
A highly reliable and a zero explosion flashlamp with a zero failure insulation system is therefore needed now that can operate in regimes never before possible for such an otherwise mature technology.
The re-entrant seal design provides a large reservoir around the lamp lead wire to inject RTV insulation to increase hi-pot resistance and lead-wire termination support.